1. Field of the Invention
This invention relates to a lighting device, in particular a lighting device to emit a light which hardly causes eyestrain.
2. Description of the Prior Art
Eyestrain is a fatigue phenomenon caused by visual tasks. It is said that environmental factors are major causes of eyestrain and greatly dependent on the quality of illumination for visual tasks and the size, movement and contrast of visual objects. In addition to usual muscular fatigue produced in the head, neck and shoulders, eyestrain sometimes causes fatigue in autonomic nerves which may result in abnormality in the control of pupil and ciliary body. It is also said that eyestrain is a primary cause to ophthalmological diseases such as asthenopia, myopia and pseudomyopia, and recent objectives of achieving healthy and pleasant life is accompanied by great demand for the development of lighting devices which hardly cause eyestrain.
As to reports on the relationship between eyestrain and illumination, for example, K. Harada, Memoirs of the Kure Technical College, Vol.17, No.1, pp.75-78 (1981) is available. In this report, the relationship between eyestrain and color rendering properties is discussed, coming to the conclusion that lights with better color rendering properties causes less eyestrain. Our study resulted in the finding that luminous sources even with a general color rendering index (Ra) of 100, for example, incandescent lamps and good color rendering fluorescent lamps, often cause eye fatigue when used in visual tasks.
As described above, since no parameter to quantify eyestrain has been found, it has been very difficult to clearly judge whether or not a particular light tends to cause eyestrain when used in illumination.